(a) Field of the Invention
The present invention relates to a paste to be fired for forming a circuit board and methods for preparing surface-modified silver powder. More specifically, the invention relates to a paste to be fired at a temperature lower than the melting point of silver for forming a circuit board, in particular an LTCC (low-temperature cofired ceramic) circuit board which comprises surface-modified silver powder consisting essentially of metal silver particles and an oxide and/or a double oxide of a specified metal element adhered to the surface of the metal silver particles, as well as methods for preparing the surface-modified silver powder suitable for use in the paste.
(b) Description of the Prior Art
In recent years, LTCC package is going mainstream in mobile communication terminal and Bluetooth RF circuit that are proceeding toward downsizing and greater packaging density for attaining single chip mounting in the near future. In the LTCC package, passive parts are provided within a ceramic multi-layer circuit board and a semiconductor chip is mounted. This configuration makes it possible to integrate the entire RF circuit into one component.
In the LTCC package that has already become commercially practical, silver or copper is mainly used as the wiring material because of the electric characteristics. Such metal powder is kneaded with an organic binder to obtain a fired type of paste, which is used to commonly prepare the LTCC package by a process that includes screen-printing on a green sheet and firing simultaneously.
The metal powder for use in the fired type of paste as mentioned above is required to have properties such as low agglomeration and uniform particle size for higher accuracy in screen-printing and easier preparation of the fired type of paste. In addition, firing requires low heat shrinkage. We now consider the case where silver is used as metal powder. Silver powder has a melting point of 960xc2x0 C., however, the sintering of silver powder is initiated at a temperature on the order of 500xc2x0 C. and its shrinkage is initiated along with the initiation of the sintering. For this reason, in case where silver powder is used as a wiring material, one should take into consideration the difference in heat shrinkage behavior between the silver wiring formed and a member to be arranged behind the wiring. Under such circumstances, there has been proposed a technique for sufficiently increasing the sintering-initiation temperature of the silver powder and for controlling the heat shrinkage due to the sintering. The technique comprises the step of preparing a fired type of paste by incorporating inorganic oxide powder into the silver powder to thus control the heat shrinkage of the silver powder. However, this technique requires the addition of a large amount of inorganic oxide particles to the silver powder and there has been observed such a tendency that this would result in the deterioration of the electrical characteristics of the resulting wiring.
Accordingly, it is an object of the present invention to provide a paste to be fired at a temperature lower than the melting point of silver for forming a circuit board, in particular an LTCC circuit board, which can show an effect of inhibiting heat shrinkage due to sintering to the upper limit in a possible lowest amount of an oxide and/or a double oxide of a metal element by uniformly adhering the oxide and/or double oxide of a metal element to the surface of individual silver particles, without impairing any electrical characteristics of the silver powder. Another object of the present invention is to provide methods for preparing surface-modified silver powder suitable for use in the paste.
The inventors of this invention have conducted intensive studies to achieve the foregoing objects, have found that the foregoing objects can efficiently be accomplished by adhering an oxide and/or a double oxide of a specified metal element to the surface of individual metal silver particles to thus convert the silver powder into surface-modified silver powder and have thus completed the present invention.
According to a first aspect of the present invention, there is thus provided a paste to be fired at a temperature lower than melting point of silver fox forming a circuit board comprising surface-modified silver powder consisting essentially of metal silver particles having an average particle size of not more than 10 xcexcm and at least one member selected from the group consisting of oxides and double oxides containing at least one metal element selected from the group consisting of those belonging to Groups 2 to 14 of Periodic Table whose atomic number falls within the range of from 12 to 82, which is adhered to the surface of the individual metal silver particles. Such silver powder is hereinafter simply referred to as xe2x80x9csurface-modified silverxe2x80x9d.
According to one embodiment in a second aspect of the present invention, there is thus provided a method for preparing surface-modified silver powder comprising the steps of adding an aqueous solution containing at least one member selected from the group consisting of water-soluble salts of metal elements each having an atomic number ranging from 12 to 82 and belonging to Group 2 to 14 of Periodic Table to a slurry containing metal silver particles dispersed therein and then adjusting the pH of the mixture with an acid or an alkali to thus carry the metal oxide and/or double oxide derived from the water-soluble salt on the surface of the metal silver particles,
According to another embodiment in the second aspect of the present invention, there is thus provided a method for preparing surface-modified silver powder comprising the steps of adhering at least one member selected from the group consisting of ultrafine particles of oxides and double oxides containing at least one metal element having an atomic number ranging from 12 to 82 and belonging to Group 2 to 14 of Periodic Table to the surface of metal silver particles and colliding the metal silver particles carrying the ultrafine particles adhered thereto with each other or other objects to thus carry the ultrafine particles on the surface of the metal silver particles.
According to still another embodiment in the second aspect of the present invention, there is thus provided a method for preparing surface-modified silver powder comprising the steps of mixing, with heating, a suspension containing at least one member dispersed therein and selected from the group consisting of ultrafine particles of oxides and double oxides containing at least one metal element having an atomic number ranging from 12 to 82 and belonging to Group 2 to 14 of Periodic Table with metal silver particles, removing the medium of the suspension to thus carry the ultrafine particles on the surface of the metal silver particles; and colliding the metal silver particles carrying the ultrafine particles adhered thereto with each other or other objects to thus carry the ultrafine particles on the surface of the metal silver particles.